Apparatus for production of thermoplastic sheet

ABSTRACT

Plastic polyolefin sheets may be formed by passing extruded resin through a primary cooling zone having a moving, smooth free surface on the cooling liquid, cooling being effected by laminar flow cooling liquid and thence by turbulent flow cooling liquid.

[ 1 Feb. 20, 1973 United States Patent 91 North et al.

[56] References Cited UNITED STATES PATENTS [54] APPARATUS FORPRODUCTION OF THERMOPLASTIC SHEET X 0 8 H 4 6 V. 2 r m D m n u 0 u h m.mE m w m M 3 N em I e fi b no m cm N O R 1 7 9 m H m H m C 7 E y w Wean 2 r 2 .mm n 3 PA 6" T w amd n ni m wmfi n ta E Y i. a r. L B d n nN 89 a r n III e P md Nha .m mL W & 6V d d m r r aWa D. wrw Wm 0 d vS0 HHEEC g m t w m v y .m A 1 5 3 7 7 .l .l.

[57] ABSTRACT Plastic polyolefin sheets may be formed by passing ex- 22Filed: April 16,1971

21 Appl. No.: 134,598

truded resin through a primary cooling zone having a moving, smooth freesurface on the cooling liquid, [52] U.S. 264/180 cooling being effectedby laminar flow cooling liquid [51] Int. Cl. and thence y turbulent flowi g liquid 1 Claim, 1 Drawing Figure [58] Field of Search is K mPATENTED 3,717, 425

EBKap/an HLi/Vorfh INVENTORS HWShe/Q/ BY ATTORNEY APPARATUS FORPRODUCTION OF THERMOPLASTIC SHEET BACKGROUND OF THE INVENTION Thisinvention relates to a process for the formation of thermoplastic sheet.More particularly, it relates to the high speed formation of polyolefmsheet.

As is well known to those skilled in the art, thermoplastic compositionssuch as polyolefins (i.e., polyalkylenes) including polyethylenepolypropylene, have been formed into various structural shapes andforms. It has heretofore been common to form polypropylene sheet inparticular into film by orientation, either monoaxially or biaxially, bystretching at controlled temperature; and the product film,characterized by high physical strength, clarity, high impact strength,and superior electrical properties has found application in a widevariety of end uses.

It has, however, heretofore been found that the formatiori ofthermoplastic sheet suitable for orientation at high rates may bepossible only with film or thin sheet, i.e., having a thickness lessthan about ten mils. Commercial production of oriented polyolefin,typically polypropylene film has included the steps of extruding a sheetof desired thickness from a charge material, normally polypropylenepellets. The sheet may be quenched from its high extrusion temperatureas by contact with water or a chill roll to reduce it to desiredtemperature, typically 20 C. to 70 C., say 40 C. The sheet may then beheated to orientation temperature (for polypropylene) of 110 C. to 165C., say

145 C. (for biaxial stretching or 130 C. (for monoaxial stretching) andthen be stretched either monoaxially or biaxially (simultaneously orsequentially) as desired.

Proper operation of the process requires that the extruded sheet bequenched rapidly and promptly after extrusion to minimizecrystallization and with a minimum of preorientation.

It is well known that the step of quenching the heavy gauge polyolefinsheet following extrusion is rate determining. Thus, it is extremelydifficult to produce heavy gauge sheet suitable for orientation, havinga thickness greater than mils, particularly above mils, at the desiredhigh rates of production. Accordingly, for this reason, high speedproduction of oriented polyolefin film of required gauge and degree oforientation (i.e., stretch ratio) has not heretofore been commerciallypossible.

It is an object of this invention to set forth an apparatus forpreparing thermoplastic sheet. Other objects will be apparent to thoseskilled in the art on inspection of the following description.

SUMMARY OF THE INVENTION In accordance with certain of its aspects, thenovel method of this invention for producing a thermoplastic film maycomprise:

a. forming a thermoplastic resin in the form of a molten, thermoplasticsheet body;

b. passing said sheet body downwardly through cooling liquid in aprimary cooling zone;

c. maintaining a moving, smooth, free surface on the cooling liquid insaid primary cooling zone at the point at which said sheet body enterssaid primary zone;

d. withdrawing said sheet body through an exit in the lower portion ofsaid primary cooling zone;

and

e passing primary cooling liquid in cocurrent laminar flow in contactwith said sheet body as it passes through said primary cooling zone;

f withdrawing a stream of primary cooling liquid with said sheet bodythrough said exit in said primary cooling zone;

g passing said sheet body through a relief section, which is without afree liquid surface, in a liquid discharge zone;

h passing said sheet body from said relief section zone into a secondarycooling zone;

i passing secondary turbulent liquid at turbulent flow counter currentto said sheet body as it passes through said secondary cooling zone;

j passing said turbulnet flow secondary cooling liquid from saidsecondary zone to said relief section at a point slightly below andsubstantially colinear to the exit of said primary cooling zone;

k maintaining the liquid level in said primary cooling zone by the rateof flow of said withdrawn stream of primary cooling liquid and inaccordance with the flow of turbulent flow secondary cooling liquidexiting said secondary zone; and

l withdrawing said sheet body from said secondary cooling zone.

DESCRIPTION OF THE INVENTION The thermoplastic resins which may beemployed in practice of the process of this invention may typicallyinclude polyamides (e.g., nylons), polyolefins including polypropyleneor low density or high density polyethylene, etc.

The polyolefin compositions which may find use in practice of thisinvention may include polymers including homopolymers and copolymers ofalpha olefins such as ethylene, propylene, butene-l etc. The preferredhomopolymers may be polypropylene and polyethylene. The polyethylenewhich may be employed may preferably be high density polyethylenetypically having a melting point of 127 C. to 135 C., an average densitygreater than about 0.92, typically 0.95 to 0.98 in its annealed state,and a degree of crystallinity of typically percent to percent.

The preferred polymer which may be used in the practice of thisinvention may be polypropylene, typically crystalline polypropylenehaving an n-heptane insoluble content of 84 percent to 96 percent and amelt flow of 0.5 to 12.0 Commercial crystalline polyalkylenes,preferably polypropylene including copolymers may be employed whereinthe copolyrnerization does not substantially interfere with the closepacking of the monomers necessary to give high crystallinity.

In practice of the process of this invention to prepare a thermoplastic,e.g., polypropylene sheet having a thickness greater than about 10 milsand typically 15 60 mils, thermoplastic resin pellets may be formed intoa molten thermoplastic sheet body by extrusion. The extruded sheet maybe rapidly quenched to form a solid sheet body. This may be effected byextruding, as from a screw-type extruder under conditions minimizingcrystallinity and pre-on'entation in the sheet. Commonly, this may beachieved by extrusion at a pressure of 2000 to 6000 psig, say 4000 psig,at a temperature (for polypropylene) of 200 C. to 320 C., say 225 C., or(for polyethylene) of C. to 30 C., say 200 C or (for nylon) of 230 C. to270 C., say 240 C.

In one illustrative mode of operation, a sheet of polypropylene (40 milsthick) having properties particularly suited for subsequent orientationto form simultaneously biaxially oriented polypropylene may be producedby feeding polypropylene pellets (having a melt flow of 5 to 7 and ann-heptane insoluble content of 92 percent to 95 percent) to a screw-typeextruder. The pellets may be melted and homogenized to form a uniformmelt at 200 C. to 320 C.; and this melt may be extruded at 2000 psig to6000 psig, say 4000 psig.

The polypropylene melt may be formed into a molten flat sheet form by aflat die with an opening 30 to 80 mils, say 60 mils, at a typical rateof 90 pounds per hour per inch of die.

Quenching from the melt temperature, through the crystallization zone,is done in two steps first by a relatively short, low heat transfer zonecharacterized by a smooth liquid surface, followed directly by aturbulent, high heat transfer zone wherein most of the heat of extrusionis removed.

The formed, molten thermoplastic sheet body exiting the extruder may bepassed downwardly through an air gap into a cooling zone. Preferably,the air gap may be very short or thin, commonly having a length (i.e.,height) of less then 25 mm, typically 1 mm to 10 mm, say 5 mm. It is afeature of this process that the air gap be of minimumlength. The 60 milthick thermoplastic molten resin sheet body, moving downwardly attypical velocities of 25 to 200 fprn, say 65 fpm, may pass through theair gap in 0.001 to 0.08 second, typically 0.015 second.

The sheet body may be passed from the air gap to the cooling zonewherein it may be contacted with cooling liquid, typically water, toquickly cool it to desired temperature. The sheet body first enters aprimary, cooling zone in which there is maintained a smooth, moving,free surface on the cooling liquid at least at the point at which thesheet body enters this zone. The primary cooling zone (and also thesecondary zone and relief section noted infra) may include a pair ofsub-zones, one on each face of the downwardly moving sheet body. Eachprimary cooling sub-zone may contain a liquid inlet, preferably at alower outer portion thereof, which projects liquid through a courseincluding a path horizontally across the upper portion of the primarycooling zone toward the point at which, i.e., the line along which, thesheet body enters the primary cooling zone. The velocity of horizontalflow across the surface may be 0.5 to 5 fprn, say 2 fpm, which issufficiently low to maintain laminar flow and to maintain amoving,smooth, free surface on the cooling liquid at least at the pointat which the sheet body enters the primary zone.

As the primary cooling liquid in the primary cooling zone approaches thedownwardly moving sheet body, it is drawn along therewith in cocurrentlaminar flow. The sheet body leaves the primary cooling zone through anexit in the lower portion thereof; and primary cooling liquid may passalong therewith. Typical travel of the sheet body through the primarycooling zone may be 5 cm to cm, say 10 cm.

Typically, charge cooling liquid may be added to the primary zone atcontrolled rates at temperature of 5 C. to 80 C., say 40 C. Coolingliquid may leave the primary zone at 10C. to 90 C., say 45 C.

The exit from the primary cooling zone may typically be 4 to 30 mm, say8 mm greater in total width than the thickness of the sheet and thethermoplastic film body may pass therethrough. Typically, thetemperature of the film body at this point may be a mean average 10 C.to 30 C., say 20 C., less than the temperature at which it entered theprimary cooling zone. For

polypropylene, the exit temperature at the surface of the film body maybe 160 C. to 200 C., say 180 C.; at the center of the film body, thetemperature may be 200 C. to 300 C., say 225 C.; and at the thermal mean180 C. to 250 C., say 210 C.

Because of the controlled relationship between the dimensions of theexit from the primary cooling zone, the length of the relief section,the flow rate in the primary zone and the velocity of secondary coolingliquid, it is possible to achieve a smooth, free surface in the primaryzone and to control the level required to achieve the short air gap; andthis level will be higher than the liquid level in the liquid dischargezone.

The sheet body exiting the primary cooling zone may be passed into arelief section. This relief section may be slightly below and colinearwith the primary cooling zone exit; and the liquid passing through thisexit may be controlled by the flow rate to theprimary zone. The reiiefsection may be a submerged zone, without a free surface, through whichthe cooling liquid from the primary and secondary zones leaves the faceof the downwardly moving film body.

The cooling liquid may then pass into the liquid discharge zone and movelaterally thereacross to be discharged therefrom through submergedopenings or over a weir adjacent to thefree surface of the liquiddischarge zone.

The relief section is short and therefore contributes little to coolingof the sheet body. Typically, the sheet temperatures are reduced about10 percent as much as they are in the primary cooling zone.

The downwardly descending sheet body may pass from the relief sectioninto an attenuated secondary cooling zone. In this zone, the sheet bodymay be contacted with a stream of cooling liquid, moving at turbu-- lentflow, which passes through the attenuated secondary cooling zone.Cooling liquid may be released from the secondary cooling zone throughopenings in the sides thereof which may be closed off. In this zone, thedownwardly descending sheet body may pass through the crystalline pointand through the temperature range in which the crystalline growth ismaximum. Commonly, the cooling liquid may enter this secondary zone at 3C. to C., say 5 C. and, as it flows countercurrently to the downwardlymoving sheet, it cools the thermoplastic sheet body and its temperaturemay rise by 0.5 C. to 5 C., say 2 C. The secondary zone may typically bean attenuated passage extending 5 to 25 mm, say 12 mm, on each side ofthe sheet body; and cooling liquid may pass therethrough at a velocityof 50 to 400 fprn, say 200 fpm. The effective heat transfer coeflicientin the secondary or high liquid velocity cooling zone may typically beto 400 BTU/hr. ft. F., say 300 BTU/hr. ft. F.

The turbulent flow secondary cooling liquid leaving the secondarycooling zone may be passed to the relief section and at a point slightlybelow and colinear with the exit slot from the primary cooling zone. Thepressure generated by the upward jetting streams of this cooling liquidleaving the secondary zone may effect control of the static head in theprimary cooling zone. The liquid level in the primary zone may also becontrolled by the back pressure contributed by the liquid level in theliquid discharge zone, by the velocity components of the cooling liquidas it moves across the relief section, by the width of the restrictorexit slot in the primary cooling zone, and by the liquid flow rate tothe primary cooling zone. The liquid outlets for liquid leaving theliquid discharge zone preferably include a plurality of openings orrelief ports in the outer walls and a top weir.

The downwardly moving sheet body may leave the secondary cooling zoneafter a typical travel therein of 0.2 to 2 meters, say 0.6 meters over0.2 to seconds, say 1.2 seconds. As the sheet body leaves the secondarycooling zone (at a surface temperature of 4 C. to 50 C., say 40 C., acentral temperature of 80 C. to 150 C., say 120 C. and a thermal meantemperature of 50 C. to 130 C., say 90 C.), it may be contacted withfresh secondary cooling liquid.

Preferably, the sheet body, after leaving the secondary cooling zone,may pass through a plenum zone and then leave the cooling system througha seal.

It is a particular feature of the novel process of this invention thatthe product thermoplastic resin sheet body attained may be characterizedby outstanding properties which may be readily controlled. The novelcooling system permits attainment of product sheet which is free ofundesirable blemishes or surface defects and is characterized by loworientation and either low or high density as desired for its subsequentuse.

It is possible to control the density of the product polymer film bycontrol of the various conditions. For example, in one pair ofcomparative runs, the density of polypropylene sheet was unexpectedlyreduced from 0.892 to 0.887 g/cc. (a significant change is 0.001 0.002g/cc.) by increasing the water velocity in the secondary zone by afactor of 2.5.

It is also possible to significantly increase the overall rate of heattransfer for cooling the film body to a value of 250 t0 400 BTU/hr. ft.F. which is about 140 percent to 420 percent greater than thatattainable by prior art techniques.

The sheet so prepared may be further treated, as by further cooling, andthereafter used, for example, in vacuum forming processes or it may besubjected to orientation processes, or it may be used in cable wrappingoperations.

In accordance with certain of its other aspects, this invention relatesto a cooling cell for containing cooling liquid, particularly adapted tocool a thermoplastic sheet body moving therethrough, which may comprise:

a. an upstanding cooling cell container including opposed upstandingouter longer wall portions, a bottom wall portion having an inlet slottherein through which said thermoplastic sheet body may move, andopposed upstanding inner shorter wall portions terminating at theirlower ends at said inlet slot and defining therebetween an upstandingsecondary cooling zone extending from said inlet slot to an outlet slotat the upper end of said inner shorter wall portions;

b. an upstanding primary cooling zone container, at least a portion ofwhich is surrounded by the upper extremity of said outer longer wallportions of said cooling cell container, including upstanding enclosingwall portions, a bottom wall portion having an exit slit therein throughwhich said thermoplastic sheet body may move, said exit slit beingpositioned above and colinear with said outlet slot and defining arelief zone between said outlet slot and said exit slit; c. means forforming a molten sheet body of thermoplastic resin and for directing itthrough said upstanding primary cooling zone, said exit slit thereininto said relief zone between said primary and said secondary coolingzones, said outlet slot of said secondary cooling zone, said secondarycooling zone, and said inlet slot of said secondary cooling zone; (1.means for passing primary cooling liquid to said primary cooling zone,across the upper free surface thereof as a moving, smooth free surfacetoward said sheet body, concurrently therealong, and through said exitslit; I means for passing secondary cooling liquid to said inlet slot,through said secondary cooling zone, and through said outlet slot intosaid relief section; f. means for passing at least a portion of saidsecondary cooling liquid against said exit slit thereby controlling theheight of the level of cooling liquid in said primary cooling zone;

g. means for withdrawing said sheet body from said inlet slot; and

h. means for withdrawing liquid from said cooling cell at a level belowthe liquid level of said primary cooling zone.

The upstanding cooling cell container which may be used in practice ofthe process of this invention may include opposed upstanding outerlonger wall portions enclosing a liquid discharge zone having arectangular horizontal cross-section and which commonly may be 0.2 to0.4 meters, say 0.3 meters broad, 2.0 to 6.0 cm, say 4 cm wider than thesheet width, and 0.21 to 2.5 meters, say 0.65 meters high (parallel tothe moving sheet body). There may be an opening or conduit at the lowerextremity thereof for withdrawing cooling liquid. Opposed end walls maydefine the cooling cell. The bottom wall portion may preferably includean inlet slot through which a thermoplastic sheet body may move.Typically, the slot may be 10 to 50 mm, say 25 mm wide and 2 to 6.0 cm,say 4 cm broader than the sheet width.

Affixed to the bottom wall portion may be opposed upstanding inner wallportions which are shorter than the outerwall portions. Typically, theinner wall portions may be 0.2 to 2 meters, say 0.6 meters. The innerwall portions may terminate at their lower ends at the inlet slot andmay define an upstanding secondary cooling zone extending from the inletslot upwardly. The secondary cooling zone terminates, at its upper end,at an outlet slot, which typically may be of the dimensions similar tothose of the inlet slot. The inner wall portions of the cooling cell(which define the secondary cooling zone) may bear a plurality of spacedopenings through which liquid may flow from the secondary zone. As theopenings are opened or closed, the velocity of liquid in the secondaryzone may be controlled; and it may be possible to vary the velocity atdifierent levels in the zone.

Adjacent to the inlet slot of the secondary cooling zone, and preferablycoterminous with the lower portion of the cooling cell may be a plenumzone through which cooling liquid may be admitted to the secondarycooling liquid zone at a uniform flow across the breadth of thesecondary cooling zone. Typically, the walls of the plenum zone may becontiguous with the outer walls of the cooling cell; and the bottom wallof the plenum zone may be spaced -30 cm, say cm, from the bottom wall ofthe cooling cell.

The bottom wall of the plenum zone may bear an outlet seal, typicallyopposed strips of a flexible resilient composition, such as Teflon,which may be mounted on the bottom wall by a clamping strip. The outletseal may permit passage therethrough accompanied by little or no coolingliquid.

An upstanding primary cooling zone container, typically 5 to 15 cm, say12 cm high, 1 to 4 cm, say 2.5 cm wider than the sheet width, and 0.1 to0.3 meters, say 0.2 meters broad may be mounted in the upper portion ofthe cooling cell above the outlet slot of the secondary cooling zone.The lower part of the enclosing wall portions and the bottom wallportions of the primary cooling zone may be below the upper extremity ofthe outer longer wall portions of the cooling cell container.

The bottom wall portion of the primary zone container may contain anexit slit in the control portion thereof. Typically, this slit may be Ito 4 cm, say 2.5 cm longer than the sheet width and 5 to 30 mm, say 8 mmwider than the sheet thickness. Preferably, it will be aligned with theoutlet slot of the secondary zone whereby moving sheet body may readilypass from one to the other and cooling liquid from one may jet againstthe cooling liquid 'from the other. Preferably, the closest portions ofthe exit slit from the primary zone and the outlet slot from thesecondary cooling zone may be 0.5 cm to 4 cm, say 1.2 cm apart; and thismay be one of the dimensions defining the relief section of the primaryzone.

The primary cooling zone zone; may include liquid inlets which may, forexample, be in the form of sparger openings positioned within a body ofpacking enclosed within a baffle box to insure uniform quiescent flow.The openings may preferably be in liquid conduits laying in the lowerportion (one on each side of the descending sheet body) of the primarycooling zone, directing flow of cooling liquid away from the film body,toward the outer wall portion of the primary cooling zone, then upwardlytherealong, and then across the face of said primary zone, as a moving,smooth, free surface. The surface of the cooling liquid may movelaterally at a velocity of l to 5 fpm, say 2 fpm, which will besufficient to minimize build-up of any local hot areas and which willmaintain the surface free of undesirable turbulence.

DESCRIPTION OF A PREFERRED EMBODIMENT The novel process of thisinvention may be carried out in accordance with certain specificembodiments thereof by the technique shown in the drawing whichschematically illustrates in cross section one apparatus in which theprocess may be carried out.

In this embodiment, thermoplastic composition may be extruded throughdie 10 having outlet 11 having a 60 mil die gap sheet to produce body12. The sheet body 12 passes through air gap 13 which in this embodimentmay be 5 mm high. The molten thermoplastic sheet body 12 may pass tocooling cell generally designated 14. Cooling cell 14 may include aliquid discharge zone generally designated 15, a secondary cooling zonegenerally designated 16, a primary cooling zone generally designated 17,a relief section generally designated 27 and a plenum zone generallydesignated 28.

Cooling cell container 14 like primary and secondary cooling zonecontainers and plenum zone containers may each be preferablybi-sectional, each of the two essentially similar sections being adaptedto face upon and to cooperate with one face of sheet body 12.

Cooling cell 14 may include upstanding outer longer walls 19 each havingat the upper portion thereof a plurality of vertically spaced opening 20(which may be stoppered) and an overflow weir 21. In this embodiment,the walls 19 may be 0.675 meters high. Each wall 19 may have thereindrain conduit 22 at the lower extremity thereof and these are normallyclosed. Bottom wall portion 23 may traverse the bottom of the coolingcell 14, and may include an inlet slot 24 which in this embodiment maybe 25 mm in total width and 0.65 meters long. Extending upwardly frominlet slot 24 may be upstanding inner shorter wall portions 25 whichextend from inlet slot 24, which they may embrace and define, upwardlyto outlet slot 26 which may have the same dimensions as inlet slot 24.The upper end of walls 25 may terminate below the upper end of walls 19.Walls 25 may each include seven rows of 12 mm holes, each row equallyspaced over the upper half of the wall.

The upper terminating end of walls 25 including outlet slot 26 maydefine the lower end of the relief section, generally designated 27.

The lower section of the cooling cell 14 includes plenum zone generallydesignated 28. The plenum zone 28 may be defined by plenum bottom wall29 having therein a flexible seal 30 supported by support strip 31.Plenum side wall 32 may be l5 cm high and may have cooling liquid inlets33. Plenum zone 28 may nave internal baffles for distribution of liquidentering through inlets 33.

Mounted in cooling cell 14 12 mm above the upper end of wall 25 may beprimary cooling zone containing 17 which includes upstanding enclosingwall portions 34, 12 cm long, and bottom wall portions 35 including exitslit 36 through which the sheet body 12 may pass. The slit 36 may be 9.1mm in total width and 0.64 meters long. The lower face of wall 35 may bepositioned 12 mm above the upper extremity of upstanding wall 25, thusdefining relief section 27. The primary cooling cell 17 may containliquid inlet conduit 37 which may deliver cooling fluid into zone 17which may contain baffle system 38 which uniformly distributes the flowalong the length of zone 17 and directs the flow of liquid upwardlyalong wall 34 across surface 39.

In practice of one embodiment of the process, crystalline polypropylenehaving a density of 0.90 a melt flow of about 5 7 and a n-heptaneinsolubility content of 92 percent to 95 percent may be extruded at arate of pounds per hour (per inch of die width) from extruder 10operating at 4,000 psig and 227 C. The sheet 12 may be 24 inches wideand 60 mils thick. Extruded sheet 12 at 227 C. may leave orifice 11 andpass through air gap 13, which is 5 mm long, at a velocity of 65 fpm.

The molten thermoplastic polypropylene sheet body may pass throughmoving, smooth, free surface 39 in primary cooling zone 17. Chargeprimary cooling liquid at 38 C. may be admitted through conduit 37 inamount of 4,800 pounds per hour and may move laterally, then upwardlyalong wall 34 and thence at a velocity of 2 fpm across surface 39 whereit may con tact sheet body 12. The cooling liquid may flow cocurrentlywith the sheet body 12; and both may exit primary cell 17 through exitslit 36. At this point, the sheet temperature may be 180 C. at thesurface, and 224 C. at the center, i.e., approximately 208 C. average.The exiting cooling liquid may be at temperature of 43 C.

The descending sheet body 12 may pass through the relief section 27wherein it may be cooled to an average temperature of 206 C. The sheetbody 12 may then pass through outlet slot 26 to secondary cooling zonewherein it may be cooled to 38 C. at the surface and 121 C. at thecenter, i.e., approximately 88 C. average, as it contacts 120,000 poundsper hour of cooling liquid moving countercurrently at a velocity of 200fpm. Cooling liquid may enter at 4 C. and exit at 5 C.

The cooling liquid leaving outlet slot 26 may pass into relief section27 wherein it exerts a jet force against exit slit 36 thereby producinga pressure at the outlet of exit slit 36 and thereby controlling liquidlevel 39.

Downwardly moving sheet passing through secondary cooling zone 16 mayexit therefrom, pass through plenum zone 28 wherein it may be cooled anadditional C. and thence through outlet seal 30.

Product polypropylene of 40 mil thickness may then pass through anadditional cooling step to reduce and equalize its temperature to 40 C.and then readily be recovered at the rate of 100 fpm and subjected tofurther treatment as desired, for example, biaxial orientation toproduce biaxially oriented film product.

Although this invention has been illustrated by reference to specificembodiments, it will be apparent to those skilled in the art thatvarious changes and modifications may be made which clearly fall withinthe scope of this invention.

What is claimed is:

1. Apparatus particularly adapted to form and cool a thermoplastic filmbody which comprises:

a. an upstanding cooling cell container including opposed upstandingouter longer wall portions, a bottom wall portion having an inlet slottherein through which said thermoplastic film body may move, and opposedupstanding inner shorter wall portions terminating at their lower endsat said inlet slot and defining therebetween an upstanding secondarycooling zone extending from said inlet slot to an outlet slot at theupper end of said inner shorter wall portions;

b. an upstanding primary cooling zone container, at least a portion ofwhich is surrounded by the upper extremity of said outer longer wallportions of said cooling cell container, including upstanding enclosingwall portions, a bottom wall portion having an exit slit therein throughwhich said thermoplastic sheet body may move, said exit slit beingpositioned above and colinear with said outlet slot and defining arelief zone between said outlet slot and said exit slit;

c. means for forming a molten sheet body of thermoplastic resin and fordirecting it through saidv upstanding primary cooling zone, said exitslit therein into said relief zone between said primary and saidsecondary cooling zones, said outlet slot of said secondary coolingzone, said secondary cooling zone, and said inlet slot of said secondarycooling zone;

d. means for passing primary cooling liquid to said primary coolingzone, across the upper free surface thereof as a moving, smooth, freesurface toward said sheet body, cocurrently therealong, and through saidexit slit;

. means for passing secondary cooling liquid to said inlet slot, throughsaid secondary cooling zone, and through said outlet slot into saidrelief section;

f. means for passing at least a portion of said secondary cooling liquidagainst said exit slit thereby controlling the height of the level ofcooling liquid in said primary cooling zone;

means for withdrawing said sheet body from said inlet slot; and

h. means for withdrawing liquid from said cooling cell at a level belowthe liquid level of said primary cooling zone.

